The focus of much study, watercraft stability is central to the proper design of all vessels. The obvious need for stability influences all decisions regarding shape, location and weight of the many components required to produce a practical and safe watercraft. The two primary aspects of stability that need to be addressed when designing a vessel are fore-and-aft stability (pitch) and side-to-side stability (roll). Pitch stability does not pose as thorny a design problem as roll stability, since each of the buoyant parts of vessels have greater length than width.
The most influential factor affecting roll stability is form stability, i.e., the shape or distribution of the buoyant component(s), such as the hull. A vessel's form stability is a product of its center of gravity (CG), and its center of buoyancy (CB) such that they result in a righting moment that must be able to resist the capsizing forces of wind and/or sea conditions.
There are three common hull forms in use today: monohull, catamaran and trimaran. The resulting moments of each design tend to keep the vessels in an upright position. All hull designs use a combination of factors to ensure roll stability in most conditions.
Any vessel can occasionally expect to encounter severe wind and sea conditions that challenge and overcome both the skill of the crew and the design of the vessel. In such situations a breakup or capsizing of the vessel can occur. Capsizing is not necessarily a catastrophic situation for small craft. There have been many recorded incidents of small craft that have capsized at sea in which the vessel has righted itself and/or the crew has survived long enough to be rescued.
To date, only ballasted monohull sailing craft or specialized, monohull lifesaving powercraft are known to have the ability to right themselves from capsizing, owing to the location of their CG, the centralized location of the CB and the omission or small size of hatches in the hull. Small, lightweight monohulls and multihulls, without interior accommodations, usually have the ability to be righted by their crew. Large multihulls, on the other hand, resist efforts to be righted by the very same forces that provide for their upright stability. This is due to the wide beam and distribution of buoyancy, combined with the rigid construction arrangement of these craft. Even multihulls with folding amas/beams are difficult, if not impossible, to right. Usually the flooding of the main hull, the location of the amas when in the folded position, and the overturning moment of the raised mast prevent such righting. The ballasted monohull tends to roll back to the upright position, whereas the multihull resists any rolling to the upright position.
Most, if not all, monohulls over twenty feet long are dependent upon the use of ballasting for maintaining stability. In some designs, stability is augmented by the use of water ballast tanks that are mounted low in the hull. However, in using this type of stability procedure, monohulls have demonstrated their potential for sinking, in the event of a breakup or capsizing, due to their specific gravity being greater than that of water. Particularly when hatches are open, broken or non-watertight, monohulls can be inundated by large waves, which flood and sink the craft. Positive flotation techniques, such as the use of foam or air tanks, are not an answer to this problem; although they have been used with success, foam or air tanks significantly reduce the usable interior volume of a vessel.
Multihull vessels obtain their stability by distributing their buoyancy between two or three spaced-apart hulls. This eliminates the need for ballast. As a result of the lack of ballast, the multihull vessel has a general tendency to float even when flooded. The typically lightweight construction of this type of vessel also aids in keeping the craft afloat. However, a multihull can sink, if it is constructed of materials resulting in a total specific gravity greater than that of water. In such a case, though, positive flotation can be employed, in lesser quantities than in ballasted monohulls. The amas of a trimaran are ideal locations for the flotation materials; this is especially advantageous, in that the main hull storage and/or accommodations are thus not reduced.
The common way to enter the hull(s) of both monohulls and multihulls is through a water-resistant (albeit not watertight), sliding hatch or door located near the control location of the vessel, i.e., on the outer or topmost deck of the craft. However, this convenience allows this opening to submerge during capsizing. Water may also enter from a large wave. Such a construction can result in the possible compromise of the structural integrity of this primary entrance. Hence, there is a probability of flooding of the hull and the resultant sinking of the vessel. In addition, this construction reduces the possibility of righting the vessel. In the case of a monohull, the vessel may roll back upright quickly enough so that only a small amount of flooding will occur through the water-resistant hatch. However, due to the wide distribution of buoyancy, the multihull cannot quickly roll to an upright position, which will result in the flooding of the vessel.
In terms of performance, based on a strict speed-to-length ratio, multihulls require less power than monohulls of similar size to achieve equal speeds. Conversely, with equal power input, multihulls can achieve higher speeds than monohulls. This relationship is due to the reduced form resistance of the multihull's lighter, narrow hulls (in contrast to the wider, heavier form of a monohull of equal capacity and/or length).
The present invention is a watercraft design that combines the light, buoyant, and easily driven form of a multihull that can be reconfigured to produce a monohull's centralized buoyancy and resultant righting ability. The inventive design includes resistance to sinking or swamping by using properly located and designed primary entrances, in conjunction with the use of well-placed flotation material, the latter of which does not sacrifice interior usage space. The overall result is a watercraft that is faster and safer than other such vessels of similar size.